Composite polymer structures and coatings can be produced in a broad range of commercial processes. Often, such a composite or coating is formed using a polymerization reaction in which polymerizable reactants are mixed together and then cured, for example, by the addition of catalyst, heat, light, or a combination thereof. Accurate and complete curing is extremely important as the structure and properties of cured polymers strongly depend upon the extent of cure, both in amount and rate. Adhesion, abrasion, solvent resistance, and usefulness are usually diminished when a composite or coating is incompletely cured. Obviously, there is a desire to improve the curing of such compositions and even to know the extent of photocuring, for example, by a suitable monitoring procedure.
One technology in which photocuring monitoring is very important is in the fabrication of micro-electro mechanical devices (MEMS) that are becoming increasingly prevalent as low-cost, compact devices having a wide range of uses. For example, such devices can be included in for example, pressure sensors, accelerometers, gyroscopes, microphones, digital mirror displays, microfluidic devices, biosensors, and chemical sensors that would be apparent to one skilled in the art.
Cationically (acid catalyzed) photocured epoxy compositions and photoresists are commonly used to fabricate high resolution and high aspect ratio MEMS structures. Typically, such processes include depositing a layer of a photocurable epoxy photoresist composition onto a substrate. A mask is placed over the photocurable epoxy photoresist composition, which mask corresponds to a desired image or pattern. The mask blocks photocuring ultraviolet (UV) rays or other radiation in desired locations so that the photocurable epoxy photoresist composition is only selectively exposed and cured. With this type of photocurable epoxy photoresist composition, the exposing UV light will cause crosslinking or curing. Once the mask is in place, the photocurable epoxy photoresist composition was irradiated with the ultraviolet light. The portions of the photocurable epoxy photoresist composition that are not exposed will not be crosslinked or cured and are usually dissolved and removed by a suitable solvent after the exposure process, leaving the desired photocured pattern on the substrate. Verifying the extent of photocuring can be critical in determining highly effective process parameters for the fabrication of MEMS structures and devices.
Another area where the monitoring of photocuring is particularly important is with various photocurable coatings that are applied to continuous or web substrates. When a roll of continuous substrate in the form of a web is used, it should be thoroughly and evenly coated with a photocurable polymer. The substrate web is generally withdrawn from the roll and coated with a photocurable composition of appropriate reactants and photoinitiators as necessary for promoting a particular polymerization reaction. The coated web is then irradiated using a suitable light source to cause photocuring. The resulting photocured composition can be further processed in-line or rolled up for later use or further processing.
Known processes of measuring the extent of photocuring in these continuous webs generally utilize off-line methods, including non-destructive methods such as infrared or UV-visible absorption spectroscopy, and destructive methods such as solvent extraction, thermal analysis (glass transition temperature), and surface tack (for example as described in ASTM-D1640-83). All of these methods have inherent disadvantages.
A non-destructive, in-line method for monitoring the degree of photocuring is described in U.S. Pat. No. 4,651,011 (Ors et al.) in which a fluorescent material such as a dye is dissolved in a monomer, oligomer, or polymer and used to monitor the degree of curing or polymerization via fluorescence anisotropy or polarization by means of an optical inspection system.
Another method of monitoring the degree of photocuring utilizes fluorescence spectroscopy and probe molecules as described for example, by (a) F. W. Wang, R. E. Lowry, W. H. Grant, Polymer (1984), 25, 690; (b) Ramin Vatanparast, Shuyan Li, Kati Hakala, and Helge Lemmetyinen, Macromolecules (2000), 33, 438-443, and (c) Paula Bosch, Fernando Catalina, Teresa Corrales, and Carmen Peinado, Chem. Eur. J. (2005) 11, 4314-4325.
In the methods described in the noted Wang et al. publication and U.S. Pat. No. 4,651,011 (noted above), photocuring monitoring requires the use of soluble probe molecules that are not covalently bound to the resulting polymer, providing potential environmental and measurement problems called “probe bloom.” Furthermore, such methods are sensitive to changes in concentration of the probe molecules that work well only at very low concentrations.
The noted methods have been shown to be useful as photocuring monitors only in low viscosity [less than 300 cP, reference (b) above] compositions. The method described for example in reference (b) requires the monitoring of small (10-20 nm) spectral shifts in the fluorescence spectrum of the probe molecule.
Many dibenzofulvene derivatives are known in the art, as described for example in U.S. Pat. No. 3,091,651 (Soderquist et al.), U.S. Pat. No. 5,047,444 (DeVoe et al.), and U.S. Pat. No. 3,091,652 (Soderquist et al.); and in J. Org. Chem. (1987), 52, 688. Furthermore, it is known that certain dibenzofulvenes are either non-fluorescent or weakly fluorescent [see H. Stegemeyer, Ber. Bunsenges, Phys. Chem. (1968), 72, 335-344].
U.S. Pat. No. 5,633,313 (Blanchard et al.) describes a method and apparatus for using fluorescent probe molecules (such as pyrene and oxazone) to monitor curing in thermally curable acrylate polymers, which upon curing cause a shift in the fluorescence emission spectrum from the fluorescent probe molecules.
U.S. Pat. No. 5,047,444 (Devoe et al.) describes the use of a latent dibenzofulvene-based fluorophore (probe) that is added to a release coating as a curing monitor. When subjected to the curing conditions, the monitor forms an ultraviolet radiation-detectable fluorophore that is detected by irradiation using an ultraviolet light source of a particular wavelength. The ultraviolet radiation is absorbed by the fluorophore that in turn emits radiation that can be detected by suitable photosensing apparatus. The intensity of the emission can be used to determine whether proper curing has been achieved. This kind of detection requires the use of a hard-to-control reaction of latent dibenzofulvene during polymerization to form a fluorescent detectable fluorophore that could lead to incorrect determinations of curing. Furthermore, since detection is based on the total intensity of fluorescence, it has quite limited utility for monitoring the degree of curing above the gel point (that is, it has limited dynamic range) and it is subject to uncertainty due to variations in background fluorescence. Furthermore, any variation in photocurable coating thickness will have an impact on fluorescence intensity and this also can lead to an incorrect curing determination.
Thus, there is a need to provide a simple, reliable, and economical method for determining the degree of photocuring of photocurable compositions such as acid catalyzed photocurable compositions. Moreover, there is a need for a reliable fluorescence probe for monitoring the degree of photocuring, and for use as a fluorescence monitor or probe to evaluate changes in various environments that may not include photocuring.